Transcript for ISIS claims responsibility for terror attack on London Underground

We begin with the terror attack in London. The threat level raised at critical. The highest level. Is another attack imminent? It comes after a bomb exploded during rush hour on a crowded subway car. Passengers scrambling to get away describing the fireball they saw. First responders rushing to help and ISIS now claiming responsibility saying other devices are hidden on the trains. President trump's tweet and Britain's prime minister did not think it was helpful. We begin with Ian Pannell in London tonight. Reporter: In London today, from every direction, first responders swarming to the scene of the rush hour attack. And tonight, all out manhunt for those who planted this smoldering bomb on the packed subway car. The home made explosive detonated at 8:20am, ruthlessly targeting passengers going to is work, children to school. Literally within three seconds of putting your bag down, the doors just closing, we hear a loud explosion occur. I turned left and I saw the fireball surged toward my side. Reporter: Then, panic as people tried to escape. There was a woman underneath me, there was a lady saying I'm pregnant. This little boy his face got smacked into the step. There was just layers and layers of people screaming. People were just crushed and trampling each other, it was every man for themselves. Reporter: On the platform, chaos, confusion. Towards the back of the train, hushed passengers make their way out, exiting single file across the tracks. Outside, emergency responders tending to the injured. I saw one woman being wheeled out on a stretcher. Her legs were quite badly burnt and wrapped up in plastic and her hairline looked quite charred. Reporter: The police say the bomb didn't fully detonate, otherwise many could've died. As it is nearly 30 were injured. At this stage we are treating the matter as a terrorist-related incident. Reporter: Today's terror striking the heart of a middle class neighborhood. There was an attack where I live and that devastated me, and it makes us feel threatened of where we live. Reporter: And tonight, police counter terrorism and mi5 special agents are now on a massive manhunt for those who set off that bomb. Ian we know there are lots of cameras inside the subway. Investigators have been poring over surveillance video? Reporter: That's right David. Hundreds of hours of surveillance video. They have also appealed to the public to hand over any social media, tweets, film, any photographs to them as well so they can look for evidence. We are also hearing there are going to be extra armed police on the streets and the military deployed to key sites. The news that ISIS say they planted bombs across the city will surely people will sleepless likely tonight. Thank you. The London attack triggering stepped up security at home. Cities and states taking action coast to coast especially in heavily populated areas and to tourist spots. Linsey. Reporter: Good evening David. Tonight the NYPD says they are not taking any chances in the event of a copycat. Unfortunately they know this drill well and once again beefing up security including bomb sniffing dogs, increased police presence. They say no direct threats to New York City but also monitoring intelligence and asking people to remain vigilant. The governor calling for increased security at bridges, tunnels, area airports and mass transportation. New York City is of course not alone. The Los Angeles police department says they are also increasing security on buses, train and at major tourist attractions. D.C. Police are telling riders to remain vigilant. Thank U. I did mention president trump's tweet. Here's what he sent this

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